1. Field of the Invention
This invention relates in general to the design and method of manufacture of packages for semiconductor chips, and the input, output (I/O), interconnections to the chips, and more specifically to a semiconductor package that utilizes a method of manufacture, molding an encapsulant that encapsulates the semiconductor chip but allows the interconnections and the backside of the semiconductor chip to be free of encapsulant.
2. Description of the Related Art
The following three U.S. patents and one U.S. patent application relate to semiconductor chip packaging designs.
U.S. Pat. No. 5,994,773 dated Nov. 30, 1999, issued to T. Hirakawa describes a BGA package incorporating a semiconductor chip and a printed circuit interposer. The chip is wire bonded to the interposer that has BGA interconnects.
U.S. Pat. No. 6,232,213 dated May 15, 2001, issued to J. L. King and J. M. Brooks shows a semiconductor chip wire bonded to a lead frame that has BGA interconnects incorporated. The assembly is encapsulated.
U.S. Pat. No. 6,353,259 dated Mar. 5, 2002, issued to T. Sato, N. Okabe, Y. Kameyama, and M. Saito discloses a design and process for a semiconductor device with peripheral interconnects utilizing TAB tape to interconnect the semiconductor to a BGA.
U.S. Patent Application Publication, U.S. Ser. No. 2002/0033412A1 published Mar. 21, 2002, by F. Tung describes the use of Cu copper pillars on semiconductor chips, and is assigned to the same assignee as the instant invention.
The advent of VLSI technology in the semiconductor field has resulted in the demand for high-density packaging. Semiconductor packaging traditionally has three levels of package. The first level, a single chip module (SCM) is made up of a semiconductor chip attached to a substrate. A substrate and chip assembly is usually molded in an encapsulant for environmental protection. The second level of package, usually a printed circuit card, mounts and interconnects the single chip modules and has a connector system to the third level package, usually a planar printed circuit board.
Elimination of a level of package has been a driving force in electronic system design in the recent past. This reduction in packaging levels would allow for closer spacing of semiconductor chips thereby reducing signal delay times. One design currently in use is direct chip attach (DCA). In this design chips are flip-chip mounted onto a substrate, usually ceramic, and the assembly sealed in an enclosure for environmental protection. The environmental protection is required to protect the semiconductor and interconnections against corrosive elements and mechanical disturbances. The inclusion of enclosures for environmental protection results in larger packages with larger distances between semiconductor chips and thereby longer signal delays.
Several interconnection technologies have been developed for use in DCA designs. TAB tape utilizes the periphery of the semiconductor chip as does fine pitch surface mount (FPT). Inherent in these designs is that the peripheral leads increase the space required by each semiconductor chip. Again this increase in chip spacing results in longer signal delays.
The ball grid array (BGA) technology is an area array interconnect design, wherein the front surface of the semiconductor chip is utilized for an array of solder spheres used to interconnect to the next level of package. This arrangement allows for the interconnects to remain within the area of the semiconductor chip.
As dimensions of semiconductor devices became smaller an additional demand on semiconductor chip interconnects has emerged. Alpha particles emitted from solder alloys used as interconnects in BGA designs, have been shown to cause semiconductor devices to malfunction. Interconnections that contain solder alloys need to be physically spaced away from the semiconductor devices. One design approach is to include copper pillars on the surface of the semiconductor chips. This approach is disclosed in U.S. Patent Application Publication U.S. Ser. No. 2002/0033412A1 dated Mar. 21, 2002. The copper pillars are incorporated between the solder of the BGA and the surface of the semiconductor chip so as to reduce the effect of the alpha particles on the devices.
First level package designs need to address the following:                Allow for reliable interconnections to the chip surface and to the next level of package.        Protect the chip and the interconnects from chemical corrosion.        Protect the chip from physical and mechanical disturbances, (shock and vibrations.)        Allow for the addition of thermal enhancements as required by the application.        
A present design that has been shown to be capable of addressing the above demands is shown in FIG. 1 (Prior Art). A semiconductor chip 10 that has a flip chip interconnect 12 on the front surface is bonded onto an interposer 14 that has printed metallurgy to connect the chip I/Os to peripheral TAB connections 16. The assembly is encapsulated in a plastic compound 18 for environmental protection. Although this design satisfies the criteria set above it has the disadvantage of increasing the area required by the semiconductor chip by 4x˜9x, due to the use of the interposer, or first level package.